This invention relates to industrial process control instruments such as pressure transmitters, current to pressure (I/P) converters, and the like having overcurrent and reverse current protection circuits.
In industrial process control systems, overcurrent and reverse current protection circuits operate between a two-wire DC current loop and a process control instrument. These protection circuits reduce the incidence of damage or degradation to the process control instrument from excessive and reverse polarity currents from the loop. Examples of such process control instruments include pressure, temperature, flow, pH, conductivity and the like transmitters, are shown for example in U.S. Pat. No. 3,975,719, and current to pressure converters, valve actuators and the like, are shown for example in U.S. Pat. No. 4,481,967, both assigned to the assignee of the present application and incorporated herein by reference.
Although a variety of operating ranges are used, two wire transmitters and I/P's typically operate in a loop current range of 4 to 20 mA, where loop current flows in one continuous loop. Energization of the loop is typically limited to a lower energy level incapable of igniting a combustible atmosphere. Therefore, since process control instruments operate remotely from control centers, the potential drops induced in each the loop wires supplying loop current and the process control instrument are critical in determining a maximum wire length for remote operation of the process control instrument. The application of process control instruments in the industrial process industry requires careful consideration of several system design parameters including lift-off potential of the process control instrument, which is desired to be reduced. Lift-off potential is a minimum potential necessary at an instrument to ensure that the process control instrument operates properly. Reducing potential drops of the overcurrent and reverse current protection circuits, each of which increase the lift-off potential of the process control instrument, allows for an increased potential drop in the loop wire thus increasing permissible wire length.
Some instruments include a diode in series with the two wire loop to block the flow of reverse current, as taught in U.S. Pat. Nos. 3,975,719 and 4,783,659 for two wire transmitters, both assigned to the assignee of the present application and hereby incorporated by reference. The diode produces a large potential drop in the instrument when current flows in the proper direction, which significantly increases the lift-off voltage and reduces the permissible wire length.
U.S Pat. Nos. 3,975,719 and 4,783,659 also teach overcurrent protection circuits in two wire current transmitters. These Patents teach a resistor in series with the two wire loop and connected to the emitter of a transistor with one end of a Zener diode connected to a remote end of the resistor and the other end of the Zener diode connected to the base of the transistor. While this combination limits the maximum value of the loop current, a large potential drop is produced across the resistor during normal operation which further contributes to increasing the lift-off potential of the process control instrument.
There is a need to provide a protection circuit that protects a process control instrument from large and reverse polarity currents from a loop where the currents can damage or degrade the instrument, while reducing the potential drop in the loop thus reducing a lift-off potential of the instrument. Further, there is a need to provide a protection circuit which diverts substantially no current back to the loop during normal operation thus improving accuracy of the instrument, in a simple, reliable and cost effective manner. A circuit that is substantially undamaged by either a decreased impedance or a short circuit of the two wire loop is desirable.